Currently, to generate narrowband or wideband waveform for use in a wireless communication system, a single baseband building block is employed.
For Orthogonal Frequency Division Multiplexing (OFDM), information is modulated on very small adjacent carriers within an allocated bandwidth at baseband. OFDM systems are designed to reduce interference among the carriers (also called subcarriers or tones).
In an OFDM system, a stream of data to be transmitted is converted from a serial stream of binary data to a plurality of parallel streams of binary data. Such conversion is usually accomplished within a transmit buffer. The parallel binary data streams may then be quadrature amplitude modulated by mapping sets of bits to complex time domain data symbols. The transformation to the time domain is done with an inverse discrete Fourier Transform (IDFT). The characteristics of complex data symbols include that each symbol describes a two-dimensional vector with a phase and amplitude. A complex data symbol is described with an in-phase and a quadrature component.
OFDM systems interpret the modulated symbols as modulated frequency tones, which are to be transformed to a signal over time in order to be transmitted. Thus, the modulated symbols, having a baseband spectrum, are mapped to orthogonal subcarriers (also called “tones”).
To minimize interference between signals transmitted on distinct tones, the signals may be filtered. As is well known in the art, a filter may be described in the frequency domain as having various bands: a pass band; a stop band; and a transition band. The pass band may be designed to have a bandwidth similar to the bandwidth of the signal to be transmitted (signal bandwidth). The stop band is designed to limit transmission of out-of-band signals. The transition band is the range of frequencies between the pass band and the stop band. The transition band may be associated with a transition bandwidth.